Electron discharge device



Nov. 19, 1957 E. D..MARTHUR 2,813,997

ELECTRON DISCHARGE DEVICE Fild Jan. 25, 1955 2 Sheets-Sheet 2 [r7 ventcfi:

.E/merH/Vc Arthur;

M's Attorney.

United States Patent ELECTRON DISCHARGE DEVICE Elmer D. McArthur, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application January 25, 1955, Serial No. 484,049

8 Claims. (Cl. 315--5.46)

This invention relates to the translation of electromagnetic energy and, more particularly, to improved devices utilizing both space charge control of current flow and velocity modulation phenomena for the generation and amplification of electromagnetic waves.

In my copending applications, Serial No. 757,164, filed June 26, 1947, Serial No. 179,854 filed August 16, 1950, now Patent No. 2,747,087, and Serial No. 265,014, filed January 4, 1952, all assigned to the same assignee as the present invention, there are disclosed certain methods and devices for the translation and generation of electromagnetic waves. This invention represents certain novel and useful improvements upon the methods and devices disclosed therein.

In my aforementioned application, Serial No. 265,014, there are described methods and devices in which an electron stream is initially modulated at the same input frequency with both space charge control and velocity components and then directed through a substantially fieldfree drift space. In the drift space, energy imparted to the electron stream by the initial velocity modulation of the electron causes the electrons comprising the beam to become bunched. The bunching, due to velocity modulation, in combination with already present density modulation, provides a high percentage modulation of the electron beam and makes possible higher beam efficiencies than previously available from either velocity modulation or density modulation techniques alone.

his a principal object of this invention to provide electron discharge devices utilizing space charge control and velocity modulation and having improved gain characteristics and reduced input loading.

A further object of the invention is the provision of electron discharge devices having a wide variety of operating currents and frequencies.

In accord with a broad aspect of the invention there is provided an electron discharge device utilizing both space charge control of current and velocity modulation. Space charge control is provided by a triode input section in which a first grid and cathode are maintained essentially at the same radio frequency potential while insulated for direct current potential. Space charge control is obtained due to an electric field established between cathode and control grid by leakage of the grid, anode field through the control grid. Velocity modulation is provided by means of an input resonant cavity connected between the second grid of the device and either cathode or first grid. Subsequent to the input resonant cavity the electron beam enters a substantially field-free drift space where bunching of the electron beam occurs. By proper control of the potential and dimensions, the input section of the device is caused to display a negative conductance, thus minimizing input loading and allowing the attainment of high gain.

The features of the invention desired to be protected are set forth in the appended claims. The invention itself, together with further objects and advantages thereof,

vmay best be understood by reference to the following 2 specification taken in conjunction with the accompanying drawing, in which:

Fig. 1 is an illustrative embodiment of the invention in simplified schematic form;

Fig. 2 is a more specific and detailed representation of the device of Fig. 1; and

Fig. 3 is an impedance chart of the input section of the device of Fig. 1.

In Fig. 1 of the diagram there is shown, in simplified schematic form, one embodiment of the invention. The device of Fig. 1 comprises a cathode 1, a first grid 2, a second grid 3 at the input end of a field-free drift space 4, within drift tube 5, a third grid 6 at the terminal end of drift tube 5, and a collector anode 7. Input resonator 8 is connected at the input end of drift tube 5 and is coupled between first grid 2 and second grid 3. An output resonator 9 is coupled between drift tube 5 and collector anode 7 and defines an output gap 10. Operating direct current potentials are applied as shown by means of battery 11. Cathode 1 while positive with respect to first grid 2 is maintained at the same radio frequency potential as grid 2 by means of a substantially zero radio frequency impedance by-pass coupling 12.

The operation of the invention is briefly as follows. An electron beam is emitted by cathode 1. The magnitude of current flowing in the beam is determined by the accelerating potentials and the negative bias impressed upon the control grid. The electron beam is accelerated by the potential applied to second grid 3 and is density modulated by space charge phenomena between cathode 1 and first grid 2. This results from the leakage field established therein. The density modulated beam enters the input gap between first grid 2 and second grid 3 wherein each electron in the beam receives an acceleration from'the field established by input resonator 8. The acceleration may be positive or negative depending upon the phase of the input signal. The electrons comprising the beam thus enter drift space 4 which is maintained substantially field-free by means of drift tube 5. Within drift space 4 the acceleration each electron has received from the input gap causes the initially density modulated beam to bunch.

It is shown in my aforementioned copending application Serial No. 265,014 that maximum efliciency is obtained from the combined space charge control of the electron beam and velocity modulation phenomena by arranging the input electrode structure in conjunction with operating conditions to secure a predetermined phase angle between the initial current modulation component provided by space charge control and the transit time or transit angle of the electrons from the inception of space charge control to the entrance to the drift space. This optimum relationship is shown in my aforementioned application to exist when the alternating current velocity component of the electron stream lags the alternatingcurrent current component by an angle of This condition was also shown to exist when the average transit angle from cathode to drift space entrance is 21r radians.

Heretofore, electron beams for velocity modulation electron discharge devices have been supplied by diode input sections. When such velocity modulation devices are adapted to include space charge control and attain enhanced efficiency, the gain is limited by input loading. Additionally, as a practical matter diode input sections displaying a 21r transit angle are extremely difficult to obtain and require large interelectrode distances or low potentials.

According to the invention, however, there is provided an electron discharge device utilizing both space charge control and velocity modulation phenomena and having an inputv section which yields. a, negative electronic conductance in shunt with the input resonator. This permits the reduction of electronic and circuit power losses to a'very low value and resultant higher gain than heretofore available. These results are obtained by providing a triode input section, for the device of the invention, in which the high frequency impedance between the control grid and cathode is held to a very low value. Preferably to zero, whereby the two named electrodes are maintained at essentially the same radio-frequency potential during operation. To this end a large by-pass capacitor may be built into the input section of the discharge device between the cathode and control grid. In addition the intere'lectrode spacings are chosen-to meet specific requirements for electron transit time to establish the proper relationship between velocity and current components as discussed herein'before to secure optimum advantage of the combination of space charge control and velocity modulation. Accordingly, as was shown with particularity in my aforementioned application Serial No. 179,854, the average transit angle from cathode to the entrance to the drift space will be 21r radians when the unidirectional potential applied to the second grid, Eg the unidirectional bias voltage applied to the first grid, Eg and the cathode-control grid spacing in centimeters Sg are approximately related by the following relation,

where a is the amplification factor of the input section, and A is the average operating wavelength.

Further characteristics of the space charge control input portion of the discharge device of the invention may be seen from Fig. 3 of the drawing, which is a polar plot of the input admittance of a typical device constructed according to the teachings of this invention. From Fig. 3 it may be seen that by varying beam current (the result of changing operating voltages), the input admittance may be caused to assume any desired value of negative conductance, and desired phase angle. For best operation of the devices of this invention, the parameters are adjusted to have 180 phase angle, i. e., zero susceptance and negative conductance. Then, by matching the negative conductance of the input section with the positive value of input resonator conductance as nearly as possible, input loading is maintained at a very low value, resulting in high gain.

It might be expected that matching negative input conductance with the positive conductance of the input resonator to eliminate, as nearly as possible, input loading, would result in oscillations in the input section, or such narrow bandwidth as to render such a device of little use. Such is not, however, the case. Devices constructed in accord with the invention have shown very usable bandwidth, for example, 6 megacycles, at an operating frequency of 1,000 megacycles. The reason believed responsible for the unexpectedly usable bandwidth of devices of the invention is that the mechanism whereby the negative conductance input section of the invention converts unidirectional energy to radio frequency energy is dependent upon transit angle only and is thus only a linear function of frequency. This means that, as a practical matter, for the small change of frequency over the operating bandwidth, operating characteristics are substantially frequency insensitive.

In Fig. 2 of the drawing there is shown a detailed drawing of an illustrative embodiment of an electron discharge device constructed in accord with the invention. The device of Fig. 2 comprises an input section 20, a drift space 21 which maybe defined by an electrically conductive, nonmagnetic cylinder, and an output resonator 22. Input section includes an input resonator 23 including an outer conductor 24 and an inner conductor "25 between which there is inserted a slidable tuning plunger 26. Tuning plunger 26, the position of which determines the resonant frequency of resonator 23, may

comprise a plurality of circumferentially spaced metallic spring fingers 27 attached to rings 28 and 29 which are separated by insulating member 30. Rods 31 may be employed as a means of adjusting the tuning plunger 26 to secure a desired resonant frequency of resonator 23. Electromagnetic energy may be supplied to resonator 23 from a suitable source (not shown) by means of a coaxial line 32 illustrated as capacitively coupled.

A stream or beam of electrons may be supplied to the input section of the device of Fig. 2 by an indirectly heated cathode 33 which includes a cylindrical member 330 having a disc shaped end piece 34 with a thermionically emissive coating 34a thereon, and a suitably supported filament 35 therewithin. Heater current is supplied to filament 35 from a conventionally represented source of direct current 36. A disc-shaped control grid 37 having a circular aperture 38 therein across which a grid of parallel metallic wires 39 is spaced, is located in axial symmetry with cathode end piece 34 and is spaced therefrom by an annular insulating dielectric disc 40. Dielectric disc 40 comprises a substantially zero impedance path for high frequency current between cathode 33 and .control grid 37 so that the two last-mentioned electrodes although insulated from one another for direct current potentials, are maintained, during operation, at the same radio frequency potential. Dielectric disc 40, together with the peripheral annular portions of cathode end piece 34 and control grid 37 with which it is in contact forms a by pass capacitor which may conveniently be of approximately to 75 micro-microfarads capacitance and must be an integral part of the device to avoid excessive lead inductances and capacitances. Control grid 37 is in direct current contact with the inner conductor '25 of input resonator 23.

Drift space 21, which is essentially free of electric fields, is defined by a conductive non-ferromagnetic hollow cylinder portion 41 which is attached at one extremity to outer conductor 24 of input resonator 23, as indicated. Orifices 42 and 43 are provided at the input and output ends of portion 41 respectively to permit the passage through drift space 21 of the electron stream emanating from cathode 33. Across orifices 42 and 43 respectively are positioned electrodes illustrated as grids 44 and 45 formed of wires 46 suitably fastened to the end sections of portion 41.

Inner conductor ,25 of input resonator 23 which is attached to control grid 38 terminates short of the input section of hollow cylindrical portion 41 to provide a gap 48 across which the electromagnetic field within resonator 23 may extend to supply both space charge control current modulation and velocity modulation of the electron stream emanating from cathode 33. Exterior conductor 24 of input resonator 23 terminates in direct current contact with the input end of cylindrical portion 41 which defines drift space 21. Thus, input signals and the electromagnetic energy thereof are impressed across the input gap defined by control grid 37 and grid 42 at the input end of drift space 21.

Since the device of the invention must be evacuated to permit electron flow, the interior portion of inner condnctor 25 of input resonator 23 is hermetically sealed by .a suitable glass or ceramic member 71 enclosing therein the filament and cathode leads. Additionally, an hermetic seal is provided between control grid 37 and the input end of drift space 21. A solenoid winding 49 supplied by a source of direct current 50 may be positioned as shown to generate a longitudinal magnetic field for focusing the electron stream.

Output resonator 22 comprises an outer conductor 51 and an inner conductor 52 between which is inserted a slidable tuning plunger 53. Tuning plunger 53 may be constructed in a manner similar to plunger 26 and may include metallic spring fingers 54, rings 55 and 56, and control rods 57. Inner conductor 52 of output resonator 22 terminates short of the output end section of portion 41 to provide an output gap 58 whereby the bunched electron stream emanating from drift space 21 may excite output resonator 22 to oscillation. Supported from a flanged circular ring 59 attached to inner conductor 52 is an anode or collector member 60 upon which the electrons impinge after exciting output gap 58. Gap 58 is bridged by a cylindrical sealing member 61 of glass or ceramic to permit internal evacuation of the device as hereinbefore mentioned. Power may be extracted from the output resonator 22 and supplied to a desired utilization circuit (not shown) by means of a concentric line 62 which is illustrated as capacitively coupled.

To supply operating potentials to the device of Fig. 4 there is provided a conventionally represented source of direct voltage 63 having a potentiometer 64 connected thereacross. Cathode 33 is maintained at an adjustably negative potential with respect to grid 44 by means of tap 66 and anode 60 is maintained at an adjustably positive potential with respect to grid 45 by means of a tap 67. Control-grid 38 is maintained at a small negative potential with respect to cathode 33 by means of tap 68.

In Fig. 4 of the drawing there is illustrated a further feature of the invention. As discussed hereinbefore, the input section of the electron discharge devices of the invention is constructed to provide a negative conductance and a preselected transit angle in order to minimize input loading and secure, to the greatest extent possible, the advantages of combined space charge control and velocity modulation. The proper conductance and transit angle are secured by adjusting interelectrode distances and potentials as hereinbefore described. Once the beam current has been adjusted by proper voltage and electrode spacings, to produce the desired input conductance and transit angle, any change in either voltage or spacing will detract from the combined effect of space charge control and velocity-modulation. It is, however, desirable to be able to control the quality factor Q (whose definition in this instance must be the most fundamental one, or, the maximum amount of energy stored in the circuit to the energy dissipated per radian) of the input resonator. Such control may be desired in order to prevent spurious oscillations due to unpreventable feedback, or to allow the increase or decrease in tube current, to match a particular output load, without changing transit angle or causing spurious oscillations. Accordingly, in the preferred embodiment of the invention the input section of which is illustrated in Fig. 4 the interior conductor 25 of input resonator 23 is surrounded at its grid end by a hollow cylindrical attenuator member 70 made of lossy material sufiicient to reduce the quality factor of the input resona tor as desired. Thus member 70 may conveniently comprise a thin cylinder of quartz coated on its outer surface with tin chloride (SnCl). Alternatively, any of the commonly used lossy materials, such as the ferrites, may be used. Member 40 may be positioned axially along inner conductor 25 of input resonator 23 with varying axial positions to obtain any desired degree of reduction in resonator quality factor.

A further modification of the input section of the devices of the invention is shown in Fig. 5 of the drawing. In Fig. 5 all elements are the same as in Fig. 2 except that input resonator inner conductor 25 makes contact with cathode 33 rather than with control grid 37. This difference results in no different operating principle, however, for control grid 37 and cathode 33 of the device are maintained at the same radio frequency potential by capacitive coupling 40.

It is within the contemplation of the present invention that the device herein disclosed may be operated as generators of electromagnetic waves as well as amplifiers. To this end the devices of this invention may be modified in a manner well known to those skilled in the art to provide the desired feedback between output resonator 22 lation and the generation of electromagnetic waves.

It will be appreciated that, while the invention has been described by reference to specific embodiments thereof, many modifications may be made without departing from the invention. It is intended, therefore, by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secured by Letters Patent of the United States is:

1. An electron discharge device comprising a cathode for supplying a beam of electrons, a first grid electrode interposed in the path of said beam, by pass means between the peripheral portions of said first grid electrode and said cathode for maintaining said first grid electrode at the same radio frequency potential as the cathode, an anode for receiving said electron beam, shielding means interposed between said first electrode and said anode defining a substantially field free drift space to enable bunching of said electron beam, a second grid'electrode at the entrance end of said shielding means defining with said first grid electrode a first interaction gap, a third grid electrode across the exit end of said shield'means defining one boundary of a second interaction gap, input means coupled to said first interaction gap for impressing space charge control and velocity modulation upon said beam, and output means coupled to said second interaction gap for extracting electromagnetic energy from said beam.

2. An electron discharge device comprising a cathode for supplying a beam of electrons, a first grid electrode interposed in the path of said beam, by pass means located between the peripheral portions of said cathode and said first grid electrode for maintaining said cathode and said first grid electrode at the same radio frequency potential, an anode for receiving said electron beam, shielding means interposed between said first electrode and said first anode defining a substantially field free drift space to enable bunching of said electron beam, a second grid electrode at the entrance end of said shielding means defining with said first grid electrode a first interaction gap, a third grid electrode across the exit end of said shield means defining one boundary of a second interaction gap, input means coupled to said first interaction gap for impressing space charge control and velocity modulation upon said beam, and output means coupled to said second interaction gap for extracting electromagnetic energy from said beam.

3. An electron discharge device comprising a cathode for supplying a beam of electrons, a first grid electrode interposed in the path of said beam, an annular dielectric disc located between the peripheries of said cathode and said first grid electrode for maintaining said two lastmentioned electrodes at the same radio frequency potential and constituting with said cathode and said first grid electrode a bypass capacitor of substantially zero radio frequency impedance, an anode for receiving said electron beam, shielding means interposed between said first electrode and said anode defining a substantially field free drift space to enable bunching of said electron beam, a second grid electrode at the entrance end of said shielding means defining with said first grid electrode a first interaction gap, a third grid electrode across the exit end of said shield means defining one boundary of a second interaction gap, input means coupled to said first interaction gap for impressing space charge control and velocity modulation upon said beam, and output means coupled to said second interaction gap for extracting electromagnetic energy from said beam.

4. An electron discharge device comprising a cathode for supplying a beam of electrons, a first grid electrode interposed in the path of said beam, by pass means located between the peripheral portions of said cathode and said first grid electrode for maintaining said cathode and said first grid electrode at the same radio frequency potential, an anode for receiving said beam, a hollow conducting tube interposed between said first grid electrode and said anode defining a substantially field free drift space .to enable bunching of said electron beam, a second grid electrode across .the entrance end of said conducting tube electrically connected thereto and defining with said first grid electrode an input gap, a third .grid electrode across the exit end of said conducting tube electrically connected thereto and defining one boundary of an output gap, a tunable cavity resonator coupled to said input gap for impressing space charge control and velocity modulation upon said beam, and a second tunable cavity resonator coupled to said output gap for extracting electromagnetic energy from said beam.

5. An electron discharge device system comprising a cathode for supplying a beam of electrons, a first grid electrode interposed in the path of said beam and maintained at a negative unidirectional electric potential with respect to the cathode, an annular dielectric disc located between the peripheries of said cathode and said first grid electrode for maintaining said two last-mentioned electrodes at the same radio frequency potential and con stituting with said cathode and said first grid electrode a by pass capacitor of substantially zero radio frequency impedance, an anode maintained electrically positive with respect to said cathode for receiving said beam, a hollow, conducting, non-ferromagnetic tube interposed between said first grid electrode and said anode defining a substantially field free drift space to enable bunching of said electron beam, a second grid electrode across the entrance end of said tube electrically connected thereto and defining with said first grid electrode an input gap, a third grid electrode across the exit end of said tube electrically connected thereto and defining one boundary of an output gap, said tube, said second and third grid electrodes being maintained positive with respect to said cathode and negative with respect to said anode, a first tunable cavity resonator coupled to said input gap for impressing space charge control and velocity modulation upon said beam, and a second tunable cavity resonator coupled to said output gap for extracting electromagnetic energy from said beam.

6. An electron discharge device system comprising a cathode for supplying a beam of electrons, a first grid electrode interposed in the path of said beam and maintained at a negative unidirectional electric potential with respect to the cathode, an annular dielectric disc located between the peripheries of said cathode and said first grid electrode for maintaining said two last-mentioned electrodes at the same radio frequency potential and constituting with said cathode and said first grid electrode a by pass capacitor of substantially zero radio frequency impedance, an anode maintained electrically positive with respect to said cathode for receiving said beam, a hollow, conducting, non-ferromagnetic tube interposed between said first grid electrode and said anode defining a substantially field free drift space to enable bunching of said electron beam, a second grid electrode across the entrance end of said tube electrically connected thereto and defining with said first grid electrode an input gap, a third grid electrode across the exit end of said tube electrically connected thereto and defining one boundary of an output gap, said tube, said second and third grid electrodes being maintained positive with respect to said cathode and negative with respect to said anode, a first tunable cavity resonator coupled to said input gap for impressing space charge control and velocity modulation upon said beam, attenuator means within said first resonator for varying the quality factor thereof, and a second tunable cavity resonator coupled to said output gap for extracting electromagnetic energy from said beam.

7. The discharge device of claim 6 wherein the attenuator means within the first cavity resonator comprises a hollow cylindrical member of lossy material partially enclosing the input gap.

8. The device of claim 7 wherein the lossy material comprises quartz coated with stannous chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,368,031 Llewellyn Jan. 23, 1945 2,425,748 Llewellyn Aug. 19, 1947 2,444,434 Feenberg July 6, 1948 

